This invention relates to the ice disaggregation arts and, more particularly, to novel means for clearing disaggregated ice from the cutter region when the expansion factor of such disaggregated ice otherwise limits the cutting rate.
In the petroleum exploration, drilling, and production industry, it is often necessary to move and station men and equipment in relatively hostile environmental regions. In recent years, the emphasis on offshore oil production in the far north has necessitated the development of new techniques for encountering formations of encroaching ice floes and the movements thereof which threaten the stability and/or position of equipment situated therearound.
In the Arctic, offshore Labrador, and the like, large regions of the ocean are often covered by thick layers of ice. Currently, there is considerable activity in these and other frozen areas directed toward the discovery and development of sources of petroleum and other natural resources. The search for and production of these resources require operational platforms for housing equipment and personnel. These platforms are typically passively transported to their operational sites and maintained in a relatively fixed position with respect to the underwater floor by anchoring thereto and/or by the utilization of dynamic positioning techniques. It may be noted, however, that some such platforms are self-propelled. In the normal course of operation, drillstrings, pipes, and the like are extended from the platform into the earth's sub-surface for accessing and recovering natural resources such as petroleum. It is thus important to maintain the platform within a predetermined envelope in order to prevent breaking or, whenever possible, the necessity for withdrawing the extended apparatus from the ocean floor.
Platforms located in both shallow and deep waters are exposed to ice floes which sometimes float freely on the water and/or unitary ice formations which flow insidiously. The ice may be so massive that a platform is susceptible to damage or destruction as a result of forces imparted thereagainst by the moving ice. The Arctic Ocean, for example, is characterized by air temperatures ranging from -70.degree. F. to 70.degree. F., ice sheets and thicknesses between 6 and 10 feet, and pressure ridges of 10 to 100 feet. In such conditions, ice typically exhibits a compressive strength of 1,000 to 3,000 psi and tensile strength of 300 to 1,000 psi. The problems of providing the requisite magnitude of force and power necessary for engagement with and disaggregation of such an environmental threat may be seen to be formidible.
Drilling and operations platforms for use in ice covered areas may take several different forms. One such platform includes a monopod, semi-submersible design utilizing a single rotating cutter completely encircling the intermediate hull section proximate the waterline for ice floe engagement and disaggregation. The cutter is disposed between upper superstructure comprising an operations platform and a submerged hull providing flotation and storage. In this manner, only a relatively narrow profile emerges through encroaching ice layers while platform surface area is maximized and buoyancy size parameters are met, respectively, above and below the ice.
A similar operations platform, which is disclosed in detail in U.S. Pat. No. 4,104,288 entitled "Operations Vessel for Ice Covered Seas", comprises a monopod, semi-submersible drilling vessel constructed with an ice-breaking wedge and ice disaggregation apparatus, the wedge and ice disaggregation apparatus comprising an intermediate hull section. The nautical wedge facilitates transit operation in both open and ice laden waters and also achieves ice-breaking within its capability while in the operating mode. The ice disaggregation portion of the intermediate hull section includes a plurality of drums rotatably mounted in generally upstanding relationship relative to the submersible hull. The drums include an outer surface adapted for breaking, cutting and/or chipping ice engaged thereby. Preferably, a pair of drums is mounted for counter-rotation such that reaction torque is cancelled and other benefits are obtained.
In the operation of such large semi-submersible operations vessels employing correspondingly large ice engaging drum structures, clearing of the broken ice chunks creates an unprecedented problem. As the ice is disaggregated from the main ice sheet, it passes into the annulus area where, due to the disaggregation process, it expands in volume. It is believed that this increased volume is approximately one-third greater than the original volume of the uncut ice.
Within the annulus area (i.e., proximate the counter-rotating drums) the disaggregated ice particles are moved through the discharge side areas at a speed approximately that of the rotational speed of the cutters. However, once the ice chunks pass through the restricted discharge side areas, they enter the full width cleared by the cutters. In this region, there is a rapid drop in velocity resulting in the ice packing behind the particles previously cleared. When the ice is relatively thin, discharged particles can move beneath the ice sheet provided the volume cut remains low. As the ice becomes thicker and/or with an increase in the speed of the cutters and the vessel, the disaggregated ice can rapidly become packed preventing any further flow of the discharged material. The quantity discharged can be enormous. For a cutting width of fifty-three feet, fifty-five foot thick ice, and a formed cutting velocity of 1.83 feet/second, approximately 5,300 cubic feet of ice is disaggregated per second. Thus, it will be apparent that, in order for the vessel to maintain position or traverse the ice when disaggregating large volumes of ice, the discharged ice particles in the ice in the discharged area must be kept moving and directed in such a way as to aid in clearance. The means for controlling the disaggregated ice chunks' discharge paths contemplated by the present invention achieves favorable ice clearing characteristics to thereby significantly increase the maximum cutting rate beyond that which can be achieved without such discharge path control.